1. Field of the Invention
The invention relates to a drive control system for a vehicle and, more particularly, to motor torque control in intermittent step-up control over a direct current/direct current converter.
2. Description of Related Art
There is known an electromotive vehicle, such as a hybrid vehicle, an electric vehicle and a fuel-cell vehicle. The hybrid vehicle is equipped with an electric motor and an engine, and travels by using at least one of the electric motor and the engine as a driving source. The electric vehicle is equipped with an electric motor as a driving source. The electromotive vehicle may transform the voltage of a direct-current power supply, such as a battery, with the use of a direct current/direct current converter, supply electric power having the transformed voltage to an inverter, convert the electric power to alternating-current power with the use of the inverter, and supply the electric power to an electric motor.
Japanese Patent Application Publication No. 2010-283932 (JP 2010-283932 A) describes a configuration including an electric motor and a control unit. The electric motor is connected to a direct-current power supply via a direct current/direct current converter and an inverter. The control unit stops the direct current/direct current converter in response to fulfillment of a predetermined stop condition.
Japanese Patent Application Publication No. 2013-193523 (JP 2013-193523 A) describes the following configuration. A hybrid vehicle includes a first motor and a second motor. The first motor drives one of a pair of front wheels and a pair of rear wheels. The second motor drives the other one of the pair of front wheels and the pair of rear wheels. The first motor is connected to a direct-current power supply via a direct current/direct current converter. The second motor is connected to the direct-current power supply without intervention of the direct current/direct current converter.
In an electromotive vehicle including a first motor and a second motor for driving wheels as in the case of the configuration described in JP 2013-193523 A, when a direct current/direct current converter is stopped by stopping the switching operation of the direct current/direct current converter in response to fulfillment of a predetermined stop condition, there is a possibility that a loss of the direct current/direct current converter is reduced if a stop time of the switching operation is extended. However, when a change in the driving force of the first motor increases as a result of a change in the required driving force of the vehicle increases during the stop time, a change in consumption energy increases, with the result that a transformed voltage may decrease in a short period of time and a deviation from a target voltage may increase. Thus, the stop of the direct current/direct current converter is cancelled in a short period of time.
In existing control, when the vehicle driving force is kept constant in a high state even when the vehicle driving force is kept constant during a stop time of the direct current/direct current converter, an electric power consumption on the first motor side with respect to the direct current/direct current converter increases, so there is a concern that the stop of the direct current/direct current converter is early cancelled. On the other hand, when a switching operation stop time is extended by merely limiting the driving torque of the first motor during the stop time of the direct current/direct current converter, there is a concern that the driving performance of the vehicle decreases. When taking into consideration this point, it is desired to achieve both a reduction in the loss of the direct current/direct current converter and suppression of a decrease in driving performance.